1. Field of the Invention
The present invention relates to a method for the synthesis of sulfo-N-hydroxy succinimide salts, novel reduced-impurity or impurity-free salts, and novel intermediate hydroxamic acid sulfonate salts.
2. Background of the Art
Sulfo-N-hydroxy succinimides (often referred to as xe2x80x9cSulfo-NHSxe2x80x9d or xe2x80x9cS-NHSxe2x80x9d) including the acid and salt counterparts have a wide range of utility in a number of broad commercial areas, including but not limited to reagents for the manufacture of biotinylation reagents, oil well drilling agents, chemical and biological assay reagents, crosslinking agents for organic biological systems or polymer systems, side chain modifying agents, solubilizing agents, reactants, markers, and the like. The class may be generally represented by the formula below, representing the central nucleus: 
Wherein X+ is a cation and R is H or an organic group, or especially any organic group formed from a compound Rxe2x80x94OH wherein Rxe2x80x94OH is an acid, and the symbolic extraction of OH (the hydroxyl) may leave the group Rxe2x80x94, forming an ester with the remainder of the central nucleus. Examples of preferred Rxe2x80x94OH compounds are acetic acid, LC-biotin suberic acid, biotin, suberic acid, 4-[N-maleimidomethyl]-cyclohexane-1-carboxylic acid, and the like. The 2- and 3- positions on the central nucleus may also be substituted. The cation may be H+, monovalent cations (such as H+, Na+, Li+, K+, NH4+, Cs+, other inorganic cations, organic cations, etc.), or polyvalent cations (including divalent cations) in which the remaining charge is satisfied by other anions (e.g., halides, nitrates, sulfates, phosphates, etc.) or forms a bis- or tris- configuration with other sulfo-NHS anions. This class of compounds is relatively expensive, mainly because of the expensive synthetic procedures which must be taken to obtain the product. Existing synthetic procedures must not only use a large number of reagents and involve a large number of synthetic steps, but the procedures involve the use of large volumes of solvents and different solvents which must be stripped after various steps as well as at the end of the procedure. The process cost involved in recapturing and stripping of the solvents is quite significant, and with increasing environmental concerns, the requirements for avoiding release of solvents into the atmosphere have become more strict and therefore more costly.
A typical synthetic process for the synthesis of sulfo-N-hydroxysuccinimide salt is known to follow the following route:
Maleic anhydride: 
is reacted with furan: 
to form a Diels-Alder reaction product: 
This intermediate product is extremely hazardous and special precautions are. required in its handling. Workers must be protectively clothed and may even be required to wear full closure protective gear (e.g., full body suits), including self-contained helmets and at least filters if not self-contained air supplies. The crystals formed are hazardous to the eyes and are easily propelled and carried by air currents. Even removal of protective garments can be hazardous because of clinging crystalline product which can be put into the air by movement of the clothing. The Diels-Alder product is then reacted with hydroxylamine (e.g., hydroxylamine hydrochloride): 
This step is done with potassium hydroxide and methanol (precipitating potassium as potassium chloride) producing an N-hydroxy succinimide adduct: 
This product is usually washed in toluene and hexane. The N-hydroxy succinimide adduct is then reacted at the hydroxyl group. This reaction is performed by combining the adduct with phenylchloroformate
C6H5O2CCl
(which is a strong lachrymator) in various combinations of triethylamine, dichloromethane, toluene and hexane and sometimes dimethyl formamide to produce the next intermediate product: 
This intermediate product is in turn dissolved in a hydrocarbon solvent, e.g., a non-polar hydrocarbon solvent (e.g., decane) and heated to elevated temperature to form the next intermediate by removal of the protecting group. 
The temperature is elevated to about 170xc2x0 C., which is above the flashpoint for decane (46xc2x0 C.). The literature also shows the use of nitrobenzene as the solvent in this step. The reaction product tends to be a black, tarry product as result of using this commercially difficult step. t-Butyl catechol may be used as an antioxidant in this step.
This last intermediate is then reacted with sodium metabisulfite in ethanol to form the sodium salt of sulfo-N-hydroxysuccinimide, which is recrystallized from aqueous methanol, isopropanol, and washed with acetone: 
This product is produced in about 95-98% purity as an amorphous solid even after repeated purification, with clear evidence of the succinimide counterpart (the succinimide or hydrogen analog of the hydroxysuccinimide) being present in the final product: 
Overall yield of the process from the original maleic anhydride is about 25-28% theoretical, and the complete time of the process is about 50 days. Numerous solvent strips must occur, and a kilogram of product is usually produced in reaction vessels of fifty liters or more.
It therefore can be seen that the entire synthetic route is complex, has toxicity, environmental and hazard concerns throughout, is expensive, and is time consuming. Improved methods of synthesis are clearly desirable.
The salt of a sulfonated succinic acid is cyclized (e.g., with a Blanc reaction), then converted to novel sulfonated hydroxamic acids by reaction with hydroxylamine, and the novel hydroxamic acid is then cyclized (e.g., by dehydration) to the sulfo-N-hydroxysuccinimide salt. A process of forming a sulfo-N-hydroxysuccinimide by cyclizing a sulfohydroxamic acid is also described.
The synthetic procedure of the present invention comprises fewer steps, can be performed in a batch process, requires fewer solvents, and produces novel intermediates and products without similar impurities as compared to processes of previous commercial use. Fewer hazardous materials are synthesized and used, and the process may be performed in a few days (e.g., 2-5 days) as compared to the approximate 50 days used for alternative procedures. Additionally, kilogram product amounts can be produced in a five liter batch process.
One process of the present invention may be described as a synthetic process comprising the steps of:
a) cyclizing a sulfo-succinic acid compound to form a monocyclic first product, and
b) opening the ring of said monocyclic first product in the presence of hydroxylamine (e.g., from an acid complex) to form a sulfo-hydroxamic acid.
The process may more specifically comprise cyclizing a sulfo-succinic acid having the central nucleus of: 
to form a monocyclic first product, and
b) opening the ring of said monocyclic first product in the presence of hydroxylamine acid complex to form a sulfo-hydroxamic acid. The process may be performed, for example, where the sulfo-hydroxamic acid comprises a compound having the central nucleus of: 
The process may then continue towards the sulfo-N-hydroxysuccinimide wherein the sulfo-hydroxamic acid is then cyclized, as by dehydration, to form a monocyclic sulfo-N-hydroxysuccinimide, wherein the sulfo-N-hydroxysuccinimide comprises the central nucleus of: 
The cyclization (e.g., by dehydration) of the sulfo-hydroxamic acid is itself a novel process which may occur in the presence of methanol, water, acetic acid, acetic anhydride, dicyclohexylcarbodiimide and/or carbonyldiimidazole, as well as any other medium which assists or acts in the dehydration of the hydroxamic acid. The dehydration may be effected merely by leaving the hydroxamic acid in a solution of the additional material, as at room temperature in water, or at slightly elevated temperatures in water. This process may be done where opening a ring of said monocyclic first product in the presence of hydroxylamine acid complex to form a sulfo-hydroxamic acid is done in the presence of excess alcohol to produce a sulfo-hydroxamic acid ester (monoester or diester, depending upon the degree of excess) as a partial product. This ester subsequently performs in essentially the same manner as the acid in conversion to the sulfo-N-hydroxysuccinimide. The alcohol is preferably methanol, but any alcohol or even glycol may be used in this step, since the esterifying moiety is subsequently removed during cyclization or dehydration of the hydroxamic acid. Because of the relatively low number of steps and the reduced amount of residues, waste material and numbers of solvents, the alcohol or glycol is reformed during the dehydration, and this may be readily recovered. This further reduces costs by recycling the alcohol or glycol and avoiding its release into the environment.
The sulfo-N-hydroxysuccinimide, as elsewhere indicated herein, is present in the absence of the corresponding sulfosuccinimide (NH or hydrogen compounds) analogs. By further separation and control of enantiomeric components, the sulfo-N-hydroxysuccinimide may have proportions of R and S enantiomers of the sulfo-N-hydroxysuccinimide present, for example, as 100 to 55% or 100 to 50% (or more than 50%, e.g., 51% or more) R or S enantiomers, and may be present as a white, crystalline powder. The sulfo-N-hydroxysuccinimide in any of these forms may be present in the absence of sulfosuccinimide (hydrogen) analogs.
The process may begin with the appropriate sulfo-succinic acid, purchased commercially or formed by the sulfonation of succinic anhydride with sulfur trioxide. The first reagent is a compound having the basic core structure: 
wherein X+ is any cation, preferably a monovalent cation such as H+, Na+, Li+, K+, NH4+, etc. Where the term xe2x80x98core structurexe2x80x99 or xe2x80x98groupsxe2x80x99 is used, the formula includes any substitution which does not change the actual atoms and bond structure shown. That is, for example with the succinimide and hydroxamic acid, on the unsubstituted portion between or intermediate the point of attachment of the sulfonate group and the carbonyl, any substitution may be present. The term xe2x80x98having a corexe2x80x99 or xe2x80x98central nucleusxe2x80x99 of Formula I therefore includes: 
wherein R is H or any other desired substituent. For example, R may be alkyl, alkoxy, halo (I, Cl, Br, F), cyano, alkenyl, aryl (such as phenyl), etc. Likewise for R1, R1 may be hydrogen, alkyl, alkoxy, halo (I, Cl, Br, F), cyano, alkenyl, aryl (e.g., phenyl), etc. Where the terminology a xe2x80x98compound of the formulaxe2x80x99 is used, that terminology excludes any substitution not specifically included in the description, e.g., allowing only the inherently understood R1 at the position between the point of attachment of the sulfonate and the carboxy group. Likewise, the terminology of xe2x80x98a core formulaxe2x80x99 or xe2x80x98central nucleusxe2x80x99 could prevent any substitution where a double bond was inserted into the backbone of the hydrocarbon chain of the compound.
The initial reagent having the core structure of the formula: 
is cyclized. This cyclization reaction may be performed, for example, with the common Blanc reaction, in sodium hydroxide and acetic anhydride. The resulting first intermediate was a central nucleus of the formula: 
This is a known compound and has a CAS registry number. This first product is then subjected to a ring opening reaction with hydroxylamine (e.g., as an acid adduct, as with hydrochloride) in methanol, producing the novel hydroxamic acid salts: 
These are novel compounds which have not been reported in the literature. These compounds exist in many tautomeric and enantiomeric forms. The R and S enantiomeric position (xe2x80x9cchiral centerxe2x80x9d) exists at the point of attachment of the sulfonate group. The tautomeric forms include, for example: 
Enolization may also occur such as: 
All of these structural variants in the structure of the sulfo-N-hydroxamic acid precursor compounds are of course expected to be statistically or potentially present as part of any composition containing the primary sulfo-NHS compound of interest, the presence of these variants being at least partially dependent upon the environment, pH conditions or other system influences on the composition.
At least three other distinguishing aspects of the present invention are noteworthy. These aspects may be individually or jointly present in the compositions of the invention. As previously noted, the process of the prior art used to produce sulfo-NHS produced the imide analog of the sulfo-NHS compound as a by-product, that succinimide (hydrogen analog) compound being represented by the formula: 
It is typically present in amounts greater than 1.0% by weight of the sulfo-NHS compound, even after repeated purification and the attempt to produce a pure sulfo-NHS composition in the process of the prior art. Even commercial sulfo-NHS contains this impurity, and of the 2-5% impurity in the commercial sulfo-NHS composition, this succinimide compound may be the largest contaminant. The process of the present invention produces a route to the sulfo-NHS compound and class of compounds which does not produce the succinimide contaminant. Therefore sulfo-NHS compositions which do not contain the succinimide counterpart are novel and, to date, have been produced only by means of the process described in the present invention. The sulfo-NHS compounds of the present invention may therefore be characterized as novel by the presence of less than 1% by weight of the succinimide counterpart, preferably less than 0.5% or less than 0.25%, and most preferably less than 0.1% down to 0% by weight of the succinimide.
Secondly, the resulting product from the process of the prior art which produces sulfo-NHS and the commercially available sulfo-NHS materials are amorphous solids. This is thought to be a result of the particular precipitation step used in the final step of the prior art process. The continuous batch process of the present invention produces distinct, white crystals. The clearly crystalline form of the sulfo-NHS compounds produced by this process is also distinct from the amorphous solid sulfo-NHS compounds produced by the prior art process.
Additionally, the sulfo-NHS compounds of the present invention may be separated into the separate enantiomers by a chiral separator. The R and S enantiomers may be separated into more highly purified isomers. This can be very important, as the R and S enantiomers will normally be used to make conjugates for use in assays, e.g., blotting or Elisa based immunoassays, etc. An unpurified mixture of enantiomers may provide a material with only 50% activity, especially if only one of the R and S enantiomers might be reactive towards a site. By providing a sulfo-NHS composition which may selectively contain higher proportions of R or S enantiomers, up to nearly 100% of either of the enantiomers, the resultant conjugates may be created to form enhanced functional systems. This can enable systems which are tailored for their degree of activity, without having to alter the remaining portions of the composition. For example, if an assay system were provided with a nominal activity of 10, using a 50/50 mixture of R and S enantiomers, the activity might be adjusted from 0 to 20 by appropriate selection of concentrations of the R or S enantiomer which was active in a particular assay. As previously mentioned, this chiral separation may be performed in a conventional manner (such as HPLC chromatography on a chiral column).